1. Field of the Invention
This invention relates to detecting the refractive index of a fluid sample. More specifically, the invention relates to a method and device using a laser beam reflected from a capillary tube holding a fluid to determine the refractive index of the fluid.
2. Description of the Prior Art
It is known in the art (see FIG. 1) to measure refractive index using a beam 4 emitted from laser 6 which is focused by lens 8 on a micrometer diameter capillary tube 10 as described in "Simple Nanoliter Refractive Index Detector", Darryl J. Bornhop, et al., 58 Analytical Chemistry pg. 504 (1986), and incorporated by reference herein. Incident laser beam 4 is diffracted by capillary tube 10. Refractive index measurements are made by a photodetector (D1) 12 measuring the intensity of laser beam 13 transmitted through capillary tube 10. A reference beam 14 is provided from laser beam 4 by beam splitter 15, and reference beam 14 is detected by reference photodetector (D2) 16 to compensate for intensity drift by the laser 6 by an operational amplifier 17 which subtracts common-mode laser noise from the refracted light. A digital volt meter (DVM) 18 records the signal. Such devices are used to test small volume samples in the nanoliter range.
A refractive index detector such as this is one of the few universal optical detectors available for fluid samples. However, these detectors tend to be either very expensive to manufacture or difficult to adjust. Typically in these devices the laser beam is focused on the tube and passes through the tube to a detector which is on the far side of the tube from the laser source. This arrangement disadvantageously requires a lens to focus the laser beam off the center axis of the capillary tube and also requires a second reference beam from the same or a second laser. Thus this method requires careful optical alignment and is thus somewhat complex and relatively difficult to assemble. Also, it is difficult to use the prior art methods with a flow cell that has a thermal compensation feature such as an optical reference arm. Additionally the single pathlength limits the ultimate sensitivity of the device.